Accurate measurement of the quantity of water distributed to each field of irrigated farmland is becoming increasingly important to the farmer. He first must assure himself that an adequate quantity of water is provided for the crops in any given field. And, secondly, he must not waste water by overwatering. Furthermore, having accurate records of the quantity of water delivered to any given field also enables the farmer to determine the efficiency of his land leveling and efficiency of water pumping. The cost of water, particularly where the water must be pumped from wells, is a significant factor in the cost of growing crops and, therefore, one over which strict control must be maintained.
Many farmland irrigation systems in use throughout the world today employ networks of open canals, or ditches, for distributing water to the fields. The most practical way to measure water flowing through a canal section is to employ a weir. See "Irrigation" by Joseph D. Zimmerman, John Wylie & sons, Inc., New York 1966, pages 353 to 355. Although considerable measuring accuracy can be achieved with permanent weirs built into the canal system as suggested in the Zimmerman text, it often is not economically feasible to provide a permanent weir at each location where flow measurement is desired. So a need has arisen for portable weirs which can be moved from site to site.
It has been proposed to build measuring weirs into check gates, or checks, and dams which have long been used to control the flow of water through the different portions of an irrigation network. Portable versions of check gates have often taken the form of flat plates cut to fit the bottom and sidewall surfaces of the canal. These check gates are installed across the canal at an angle to the vertical with the upper face of the plate facing upstream so that the weight of water backed up behind and above the check plate holds the check in place in the canal. See, for example, "Irrigation of Agricultural Lands", Managing Editor R. C. Dinauer, American Society of Agronomy, Madison, Wis., 1967, pages 830 to 833, and the aforementioned Zimmerman text at pages 308 to 311.
As suggested in the Dinauer text, weir-like openings can and have been provided in portable check gates to provide portable weirs. One of the shortcomings of past portable weirs constructed in check gates has been a lack of accuracy and reliability over time. In order to be truly portable, the weight of the weir must be kept to a minimum so that it can be lifted and transported by field workers without the aid of a crane. But light weight has usually meant fragile construction so that past weirs, when subjected to normal rough and tumble handling in the field, have become bent, dented, and distorted in the structure providing the weir opening with the result that accurate measurements could no longer be obtained.